The present invention relates principally to the manufacture of magnetic recording media used in rigid disk drives commonly used for computer data storage. In particular, the present invention relates to a novel method for manufacturing low noise, high coercivity ("Hc") isotropic media using a heated substrate and multiple nucleation layers, including a Cr-alloy underlayer.
Future magnetic disks will be required to store increasingly high density data. The recording performance of advanced magnetic disks is commonly determined by four basic characteristics, known as PW50, overwrite, non-linear-transition-shift ("NLTS") and media noise. These characteristics are described in detail in Bertram, "Theory of Magnetic Recording", Cambridge University Press, published in 1994, incorporated herein by reference.
PW50 is the half width of the output signal (i.e. the width of that portion of a pulse from the time its rising edge reaches one half of its amplitude to the time its falling edge falls to one half its amplitude). PW50 must be minimized to achieve high recording density. Means of reducing PW50 include reducing "Mrt" (the magnetic film thickness t times remanence Mr), raising Hc, increasing hysteresis loop squareness "S*", and increasing remanent coercivity squareness, "S*rem", as described by Williams and Comstock in "An Analytical Model of the Write Process in Digital Magnetic Recording," A.I.P. Conf. Proc. Mag. Materials, 5, p. 738 (1971).
Overwrite ("OW") is a measure of what remains of a first signal after a second signal (for example of a different frequency) has been written over it on the media. OW is improved by raising S* and by decreasing Hc and Mrt.
Non-linear transition shift (NLTS) refers to an unpredictable shift in the location of the written bits, which can cause an error. NLTS can be reduced by reducing Mrt and increasing Hc.
High density media also need to have low noise. The noise performance includes read jitter and write jitter. Read jitter is primarily determined by the amount of signal available from a bit, and the noise in the channel and head, and is reduced by increasing Mrt. Write jitter is determined by the intrinsic noise of the magnetic layer and can be reduced by breaking the exchange interaction between the magnetic particles and by reducing domain percolation at the transitions between magnetic domains. This may be accomplished by spacing the grains apart from one another by a few angstroms or more, or by interposing a non-magnetic material or insulator at the grain boundaries, as described by Chen et al. in "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media," IEEE Trans. Mag., vol. 24, no, 6, p. 2700 (1988). Intrinsic media noise has been theoretically modeled by Zhu et al. in "Micromagnetic Studies of Thin Metallic Films", J. Appl. Phys., vol. 63, no. 8, p. 3248 (1988). A reduction in interparticle exchange interaction has also been tied to an increase in Hc by Chen et al. and by Zhu et al. in the aforementioned references.
There is yet another source of media noise namely, the grain noise. For media with well isolated grains, smaller grains provide sharper transitions and therefore exhibit lower media noise, both in the down track as well as the cross-track directions. The grain noise is very important for future high recording density which will have bit sizes approaching the dimensions of a few magnetic grains.
Most media made today are "oriented", i.e. Hc, Mrt and S* are higher in the circumferential direction than in the radial direction, as opposed to "isotropic" media which have the same Hc, Mrt and S* in all in-plane directions. It is believed that the performance of oriented media is poorer than that of isotropic media at high bit density especially in terms of off-track noise and transition percolation. Transition percolation is worse for oriented media where the magnetic easy axis lies preferentially in the circumferential direction. For this reason, high density media are likely to be isotropic.
Hence future high density media require high Hc and Mrt, high S*, small, well isolated grains and isotropic magnetic properties. At present, media are commonly made by using two methods.